Something I've wondered about off and on... There are "masturbation clubs" where guys go to masturbate in the presence of other guys. Sometimes, things apparently progress to a point where guys are masturbating other guys. Maybe even many other guys. And so...comes the question: doesn't this pose a real risk for transmitting diseases? If Tim masturbates Alan, who drips a bit of precum, and then Tim goes to masturbate Bob, doesn't Bob stand a risk of picking up STIs from Alan via Tim's hand?
The risk is pretty much nonexistent. First, the STD would have to enter your body and not simply touch your skin, which is highly unlikely to happen with jacking off. Second, STDs pretty much start dying as soon as the leave the body. Pretty much, mutual masturbation is probably one of the safest sexual acts you can have with another person.